Solid state radiation imaging arrays typically comprise arrays of photosensor devices coupled to a scintillator in which the incident radiation to be detected by the imager (such as x-rays) is absorbed. Light photons generated in the scintillator as a result of the absorption of incident radiation pass to the photodetector array and are in turn absorbed by the photosensor, resulting in the accumulation of charge on the photosensor that corresponds to the flux of light photons; reading the charge accumulated on respective photosensors provides a measure of the intensity of the incident radiation and the relative position on the array at which the radiation was absorbed.
In one type of high resolution photosensor array photodiodes are arranged in a two-dimensional pixel array. The respective photosensors are formed on a substrate; each photodiode comprises a body (or island) that, for example, consists of a layer of n type silicon disposed over the contact pad (or, bottom electrode); a thicker layer of intrinsic amorphous silicon that is disposed over n type silicon; and, a layer of p+ type silicon disposed over the planar top portion of the intrinsic silicon body to form a common electrode contact (alternatively, a photodiode can be fabricated with the doping types reversed from that described above). A common electrode is coupled to the photodiode via the p+ type silicon layer. A bias voltage applied across the photodiode results in the formation of an increased depletion region in the semiconductive material. Charge generated in the photodiode as a result of the absorption of light photons from the scintillator reduces the bias across the diode contacts, and the collected charge is read when a switching device in the array, such as a thin film transistor (TFT), couples the photodiode to readout electronics via an address line.
Optimal spatial resolution and contrast in the signal generated by the photosensor array is achieved when incident optical photons from the scintillator are absorbed substantially only in the photodiode directly in line with the region of the scintillator in which the optical photons were generated. Optical photons incident on the photosensor array in areas other than the photodiode, such as the TFT switching devices coupled to each photodiode or the address lines, can result in scattering or absorption of the photons that causes noise in the array.
The photodiode passivation and optical coupling layers (between the photodiode and the scintillator) typically comprise a light transmissive material such as polyimide. This passivation layer is disposed over the array, including the TFTs, the photodiodes, and the address lines. Optical photons passing from the scintillator can be scattered and pass through the transparent passivation layer into the photosensor array outside of the photodiode in the array region underlying the portion of the scintillator in which the optical photons were generated. Such scattering and absorption presents problems of increased cross-talk and noise in the array. Cross-talk reduces the spatial resolution of the array, and absorption of optical photons in TFT switching devices can result in spurious signals being passed to the readout electronics.
It is thus an object of this invention to provide a photosensor array in which non-photodiode areas of the array, e.g., the TFTs and address lines, are shielded from incident optical photons.